1. Field of the Invention
The present invention relates to the earth-boring arts. In particular, the present invention describes a method and apparatus for severing a downhole tool such as tubing, drill pipe or casing.
2. Description of Related Art
Commercial systems have been around for years to sever pipe at a selected point that becomes stuck downhole. The simplest system detonates a large mass of explosive lowered to a desired point on a wireline to rupture and thereby separate the free, upper end of the pipe string from the stuck, lower end. A better system such as described by U.S. Pat. No. 7,530,397 to W. T. Bell detonates a cylindrical column of explosive simultaneously from both ends to create a shock wavefront collision at the center. The more simultaneous the end detonations and the more uniformly homogenous the explosive column, the better the cut is.
There are a few variations on the colliding shock wave concept. One variation, represented by U.S. Pat. No. 7,104,326 to A. F. Grattan et al, uses a centrally located radial shaped charge to pre-cut the pipe before the explosive shock waves collide. Another variation, such as represented by U.S. Pat. No. 4,378,844 to D. D. Parrish et al., places a metal disc at the center of the collision point with the idea that the-metal will liquefy and form a high-pressure radial cutting jet.